Systems and methods for introducing a bone anchor

ABSTRACT

Systems and methods for a guide assembly for introducing a bone anchor to an operative target site. The guide includes an outer sleeve and an inner sleeve. The outer sleeve has a distal anchor engaging end, a proximal end, and a central passage extending from the distal end to the proximal end. The inner sleeve may be situated in the central passage of the outer sleeve. The inner sleeve is movable being between a first position and a second position. The first position permits insertion of the bone anchor in the central passage. The second position releasably fixes the bone anchor to the guide assembly.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.15/986,642, filed on May 22, 2018, which is a continuation of U.S.patent application Ser. No. 15/702,742 filed Sep. 12, 2017, now U.S.Pat. No. 10,004,544, which is a continuation of U.S. patent applicationSer. No. 15/346,336 filed on Nov. 8, 2016, now U.S. Pat. No. 9,757,166,which is a continuation of U.S. patent application Ser. No. 14/947,461filed Nov. 20, 2015, now U.S. Pat. No. 9,492,208, which is acontinuation of U.S. patent application Ser. No. 13/894,355 filed May14, 2013, now U.S. Pat. No. 9,192,415, which is a continuation of U.S.patent application Ser. No. 12/322,815 filed Feb. 6, 2009, now U.S. Pat.No. 8,439,922, which claims the benefit of priority from U.S.Provisional Patent Application No. 61/026,719 filed Feb. 6, 2008, theentire contents of which are hereby expressly incorporated by referenceinto this disclosure as if set forth fully herein.

BACKGROUND OF THE INVENTION I. Field of the Invention

The present invention relates to medical devices and methods fordeploying bone anchors to an implantation site.

II. Discussion of the Prior Art

Bone anchors are often surgically implanted into a patient as part of aspinal fixation or stabilization construct. Fixation systems are oftento aid in the stabilization of a damaged spine or to aid in thecorrection of other spinal geometric deformities. Spinal fixationsystems are often constructed as a framework stabilizing a particularsection of the spine. Existing systems often use a combination of rods,plates, pedicle screws and bone hooks for fixing the framework to theaffected vertebrae. The configuration required for each patient variesdue to the patient's specific anatomical characteristics and ailments.

SUMMARY OF THE INVENTION

In one aspect of the invention, there is provided a guide assembly forintroducing a bone anchor to an operative target site. The guideincludes an outer sleeve and an inner sleeve. The outer sleeve has adistal anchor engaging end, a proximal end, and a central passageextending from the distal end to the proximal end. The inner sleeve maybe situated in the central passage of the outer sleeve. The inner sleeveis movable being between a first position and a second position. Thefirst position permits insertion of the bone anchor in the centralpassage. The second position releasably fixes the bone anchor to theguide assembly.

According to another aspect of the invention, there is provided a guideassembly for introducing a bone anchor to an operative target site. Thebone anchor includes a housing having a pair of opposed arms. There isan outer sleeve and an inner sleeve. The outer sleeve has a distal end,a proximal end, and a central passage extending from the distal end tothe proximal end. The distal end of the outer sleeve includes first andsecond lateral projections dimensioned to engage first and secondrecesses formed in the housing. The distal end also includes first andsecond longitudinal ridges dimensioned to be received within first andsecond longitudinal grooves formed in the housing. The inner sleeve hasa distal end and a proximal end. The inner sleeve may be situated in thecentral passage and movable along a longitudinal axis between an openposition and a closed position. The closed position temporarily fixesthe housing to the guide assembly. The inner sleeve further includesthird and fourth longitudinal ridges that are received within third andforth longitudinal grooves in the housing when the inner tube is in aclosed position. In some implementations the guide assembly may includea locking element.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be more fully understood from the following detaileddescriptions taken in conjunction with the accompanying drawings, inwhich:

FIG. 1 is a perspective view of a spinal fixation construct including apair of bone anchors, fasteners and, including (by way of example only)first and second pedicle screws and a connecting element;

FIG. 2 is front view of a guide assembly of couple to a bone anchoraccording to one embodiment of the present invention;

FIG. 3 is an exploded view the guide assembly of FIG. 2;

FIG. 4 is a cross section view of the guide assembly of FIG. 2;

FIG. 5 is a cross section view of the a proximal portion of the guideassembly of FIG. 2;

FIG. 6 is a cross section view of the proximal end of the guide assemblyof FIG. 2;

FIG. 7 is a cross section rotated 90 degrees for view of the proximalend of the guide assembly of FIG. 6;

FIG. 8 is an end view of the inner guide member of FIG. 2;

FIG. 9 is a longitudinal view of the outer guide member of FIG. 2;

FIG. 10 is a side view of the bone anchor housing; according to oneembodiment of the present invention;

FIGS. 11-14 illustrate the insertion and locking of the bone anchor intothe guide assembly of FIG. 2, according to one embodiment of the presentinvention;

FIG. 15 is a side view of an alternate guide assembly, according toanother embodiment of the present invention; and

FIGS. 16-18 illustrate the insertion and locking of the pedicle screwassembly into the distal portion of the guide assembly.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Illustrative embodiments of the invention are described below for thepurposes of understanding the principles of the invention. No limitationof the scope of the invention is therefore intended. In the interest ofclarity, not all features of an actual implementation are described inthis specification. It will be appreciated that in the development ofany such actual embodiment, numerous implementation-specific decisionsmust be made to achieve the developers' specific goals, such ascompliance with system-related and business-related constraints, whichwill vary from one implementation to another. Moreover, it will beappreciated that such a development effort might be complex andtime-consuming, but would nevertheless be a routine undertaking forthose of ordinary skill in the art having the benefit of thisdisclosure. The system disclosed herein boasts a variety of inventivefeatures and components that warrant patent protection, bothindividually and in combination.

With reference to FIG. 1, a pair of bone anchors 12 are shown as theymight be utilized to form a spinal fixation construct 10 (including thepair of anchors, a rigid connecting element 14 linking the bone anchors,and a pair of fasteners 15 for securing the connecting element 14 to thebone anchors 12) for preventing movement between a pair of adjacentvertebra. The bone anchors 12 include an anchor element 16 configuredfor purchase within the bone, for example, a screw, at the implantationsite and a housing 18 for coupling the bone anchor to the connectingelement 14. The housing 18 may be fixed to the anchor element 16 or thehousing 18 may be coupled to the anchor element 16 such that the housingmay move in one or more planes relative to the anchor element 16. Thehousings 18 include two arms separated by a pair of slots whichcollectively form a generally “U-shaped” structure defining orcontaining an area dimensioned to receive the connecting element 14 andthereafter the locking screw 15 (e.g. a threaded lock screw). In someimplementations the connecting element 14 may be flexible to providesupport and stabilize the adjacent vertebrae without limiting all motiontherebetween. Also, more than two bone anchors 12 may be used to linkthree or more vertebra together (e.g. multi-level). The bone anchors 12may be positioned on both sides of the spine forming a bi-lateralconstruct. The bone anchors 12 may also be anchored in implantationsites other than the pedicles show, for example only, the bone anchors12 could be implanted in the anterolateral aspect of the vertebral body.

Turning to FIG. 2, there is illustrated an example embodiment of a guideassembly 100 operable to temporarily engage and hold a bone anchor 12and aid in the implantation of the bone anchor at a desired implantationsite. The guide assembly 100 securely grasps the housing 18 of the boneanchor 12. The length of the guide assembly is generally provided suchthat at least a portion of the assembly will extend beyond a skinincision when an attached bone anchor 12 is fully implanted in thetarget site. Thus a surgeon may manipulate the guide assembly 100 from aposition outside the operative corridor to advance and position the boneanchor in the desired location. An instrument (not shown) may beadvanced through the guide assembly 100 to engage the anchor portion 16and drive the anchor portion in to the target bone. Slots 104 may beprovided in the guide assembly 10 to further help guide a connectingelement into to the bone anchor housing 18.

As shown in FIG. 3-5, the guide assembly 100 includes an outer sleeve110 and an inner sleeve 120. The outer sleeve 110 may be generallycylindrical in shape and have a length extending from a distal end 101configured to engage bone anchors to a proximal end 103. The length isdimensioned such that the proximal end 103 will extend beyond the skinentry site when the distal end 101 is positioned adjacent animplantation target site. A central passageway 170 (as shown in FIG. 9)extends through the outer sleeve from the distal end 101 to the proximalend 103. The outer sleeve 110 may further include a pair of opposedslots 104 opening at the distal end 101 and extending longitudinallyalong the outer sleeve to a position short of the proximal end.Preferably, slots 104 have a length great enough such that the proximalend of the slot remains outside the patient entry site when the distalend 101 of the outer sleeve 110 is adjacent the implantation site.Keyholes 102 having a width greater than the width of the slots 104 maybe provided, connecting to the slots 104 at the proximal ends of slots104. The keyholes 102 allow a connecting element 14 having one or moreenlarged ends, as in FIG. 1, to be advanced into the slots 104.Together, the central passageway 170, open distal end 101, and opposedslots 104 generally divide the outer sleeve 110 into two halves or arms105.

The inner sleeve 119 includes a main sleeve 120, a sleeve extension 130,and a cap 140. In general, the inner sleeve 119 has a generallycylindrical shape extending from a distal end 121 to a proximal end 145and an interior passageway 160 (FIG. 12) extending between the distalend 121 and the proximal end 145. The length of inner sleeve 119 ispreferably such that when the distal end 121 is positioned adjacent animplantation target site, the proximal end 145 extends beyond theproximal end 103 of the outer sleeve. The inner sleeve 119 is situatedin the outer sleeve 110 and configured to translate longitudinally alonga longitudinal axis relative to the outer sleeve in order to engage aportion of the bone anchor housing 18 and temporarily fix the boneanchor to the guide assembly.

The main sleeve 120 is situated within the central passageway 170 of theouter sleeve. The inner sleeve 120 may further include a pair of opposedslots 124 alignable with the slots 104 of the outer sleeve. The slots104 open at the distal end 121 and extend longitudinally along the outersleeve to a position short of the proximal end 123 of the main sleeve.Preferably, slots 124 have a length that is the same or similar to theslots 104 in outer sleeve 110. Keyholes 122 having a width greater thanthe width of the slots 124 may be provided, connecting to the slots 124at the proximal ends of slots 124. Slots 104 of the outer sleeve 110 andslots 124 of the main sleeve 120 may have widths of approximately thesame dimension selected such that a connecting element 14 may pass alongthe slots. Together, the interior passageway 160, open distal end 121,and opposed slots 124 generally divide the main sleeve 120 into twohalves or arms 125.

The sleeve extension 130 is configured to join with the main sleeve 120and may have a generally cylindrical shape. The sleeve extension 130 hasa distal end 131 and a proximal end 133. The sleeve extension 130 has aninterior lumen that extends from the distal end 131 to the proximal end133 and forms part of interior passageway 160. Sleeve extension 130 andmain sleeve 120 are joined such that they are longitudinally fixedrelative to each other but can rotate freely about a longitudinal axisrelative to each other. To accomplish this, two or more tabs 127 aresituated at the proximal end 123 of the main sleeve and include radialridges 126. A complementary radial groove 138 is provided in an innerwall of the sleeve extension 130 and receives the radial ridges thuspreventing the longitudinal motion the main sleeve 120 and sleeveextension 130. Sleeve extension 130 is also fashioned with threadedregion 136 dimensioned to engage threads 108 situated along a portion ofthe inner wall of outer sleeve 110 (best viewed in FIG. 9). At theproximal end 133 of the sleeve extension 130 a series of tabs 137protrude laterally from a ring 134 for coupling the cap 140 via a seriesof corresponding apertures 144 in the cap.

The cap 140 is situated on the proximal end of the sleeve extension 130and provides a grip for rotating the sleeve extension 130 in order toadvance the threaded region 136 along the outer sleeve threads 108,thereby longitudinally adjusting the position of the inner sleeve 119relative to the outer sleeve 110. The cap 140 has at least two slots 142opening at the proximal end 143 for coupling a reduction extension,described below.

According to one example, the cap 140 may form part of a lockingmechanism that prevents the guide assembly 100 from disengaging from thebone screw housing 18 until a user disengages the locking mechanism. Thelocking mechanism may comprise a series of teeth 148 situated about thedistal end 143 of the cap 140 and a complementary series of teeth 108situated about the proximal end 103 of the outer sleeve 110. Accordingto one example the teeth 108 generally include a perpendicular face anda sloped face. Likewise, the teeth 148 also each include a perpendicularface and a sloped face. The teeth 108 and 148 are further arranged sothat when the cap 140 is rotated, for example, in a clockwise directionto translate the inner sleeve 119 toward the distal end 101 (to affixthe bone anchor housing 18) the sloped faces of teeth 108 and 148engage, allowing the teeth to slide past each other. When the cap 140 isrotated in the opposite direction to translate the inner sleeve 119toward the proximal end 103 (to detach the bone anchor housing 18) theperpendicular faces of teeth 108 and 148 will engage, preventing furtherrotation in that direction. To disengage the locking mechanism, the cap140 is simply pulled in a direction away from the outer sleeve. To makethe cap 140 longitudinally movable relative to the sleeve extension 130,the apertures 144 are provided with a height greater than the height ofthe tabs 137. Additionally, the cap 140 may be biased towards the mostdistal position such that the locking mechanism automatically engages asthe inner sleeve 119 is translated through the outer sleeve 110 towardthe distal end. This may be accomplished, for example, by positioning aspring 146 between cap 140 and the sleeve extension 130.

With reference to FIGS. 6-14, the guide assembly 100 securely engagesthe bone anchor 12 at the distal end. The central passageway 170 of theouter sleeve is dimensioned to receive the housing 18 at the distal end103. A cutout region, best viewed in FIG. 12, provides an access pathfor side loading the housing 18 into the central passage way 170. Oncethe housing is loaded, the inner sleeve 120 is advanced distally towardthe housing, by rotating the inner sleeve clockwise (by way of example).When the inner sleeve moves towards the distal end, distal extensions128 block the cutout region such that the housing 18 cannot be removed.

Each arm 105 of outer sleeve 110 includes a lateral projection tab 150near the distal end 101 of the outer sleeve 110. The tabs 150 aredimensioned to engage a recess 30 found on the outer wall of the screwhousing 18 as shown in FIG. 6. Slots 31 situated below and incommunication with the recess 30 allow the housing to be side loadedonto the tabs 150.

With reference to FIGS. 8 and 9, within the interior passageway 160,longitudinal ridges 166 are situated on the distal extensions 128. Eachprotrusion 166 is dimensioned to fit in a longitudinal groove 32 in thehousing 18. In addition, two grooves 162, located on the outer portionof the inner sleeve 120 and opposite of the longitudinal ridges 166, aredimensioned to receive longitudinal ridges 172 of the outer sleeve 110.A channel 164 is also provided in the outer wall of the inner sleeve120. Grooves 162 and channels 164 extend from the distal end 121 to aposition near the proximal end 123 of the main sleeve 120.

Within the central passageway 170 of outer sleeve 110, a longitudinalridge 172 is situated on each arm 105 of the outer sleeve 110.Similarly, a tab 174 is also situated on each arm 105. Each protrusion172 and tab 174 extend from the distal end 101 to a position near theproximal end 103 of the outer sleeve 120.

The tabs 174 are dimensioned to engage the channels 164 of the innersleeve 120, such that the tabs 174 will slide within the channels 164.As illustrated in FIGS. 8-9, and by example only, the tabs 174 andchannels 164 have a dovetail configuration. The protrusions 172 aredimensioned to slide within the grooves 162 of the inner sleeve 120. Theinteraction between the tabs 174 and the channels 164 and between theprotrusions 172 and the grooves 162, the main sleeve 120 may translatelongitudinally within the outer sleeve 110 while rotation of the mainsleeve 120 is prevented.

Turning to FIG. 10, each arm of the bone anchor housing 18 has twolongitudinal grooves 32, one on each end of the arm. One groove 32 ofeach arm receives the longitudinal ridge 166 of the inner sleeve 120 andthe other longitudinal groove 32 engages the protrusion 172 of the outersleeve. In addition, each arm of the housing 18 has a recess 30dimensioned to engage the tab 150 of the outer sleeve 110. As seen inFIG. 16, each recess 30 communicates with a rectangular slot 31. Theslot 31, allows the tab 150 to slide horizontally along the arm so thetab 150 engage the recess 30. Thus, when the bone anchor housing 18 isloaded in the guide assembly 100 and the inner sleeve 119 is moved intoclosed position, the housing 18 and guide assembly 100 are positivelyengaged in six locations providing a secure attachment.

FIGS. 11-14 illustrate the process of attaching the guide assembly 100to the screw assembly 12. First (if the guide assembly is in a “closed”position), as illustrated in FIG. 11, the user pulls the cap todisengage the teeth 108 and 148. The user may then rotate the cap 140counterclockwise (by way of example). Turning the cap 140 rotates thesleeve extension 130 whereby the threaded region 136 travels proximallyalong the threads 108 of the outer sleeve 110, translating the innersleeve 119 proximally relative to the outer sleeve 110. This results inthe distal end of the guide assembly 100 moving to an “open” position

Next, as shown in FIG. 12, bone anchor housing 18 is positioned in theguide assembly 100. The housing 18 is loaded into the central passageway170 through the cutout region and the recesses 30 are engaged with thelateral projection tabs 150 two grooves 32 of the housing 18 will eachreceive a longitudinal ridge therein. At this stage the housing 18 isloosely held by the outer sleeve 110 only.

As shown in FIG. 18 to fully secure the bone anchor 12 in the guideassembly, the user will rotate the cap 140 clockwise. This causes theinner sleeve 120 to translate distally relative outer sleeve 110. As theinner sleeve 120 travels distally, the longitudinal ridges 166 on distalextension 128 of the inner sleeve will each engage the remaining grooves32 of the housing 18. As the inner sleeve 119 approaches the closed orlocked position locking mechanism will automatically engage as the teeth108 and 148 meet. Once the bone anchor 12 is secured, the guide assemblymay be advanced through an incision in the patient and to theimplantation target site.

FIGS. 16-18 illustrate a reduction assembly 300 that may be utilizedwith the guide assembly 100. The reduction assembly 300 is configured toapply force to draw the housing 18 and a connecting element 14 deployedvia the guide assembly 100 together prior to introducing a fastener 15.FIG. 16 depicts the reduction assembly 300 in position over a centerguide assembly for deploying a multi-level construct. FIG. 17illustrates an exploded view of the reduction assembly 300. Thereduction assembly consists of a reduction sleeve 302, a reducerextension 340, and a reduction handle 330. The reduction sleeve 302 hasa generally cylindrical shape with an inner lumen (not shown)dimensioned to slide over the guide assembly 100. At the distal end,openings 320 at opposite ends of the reduction sleeve 302 aredimensioned to engage the connective element 14.

With reference to FIG. 18, the tabs 346 of the reducer extension 340engage the slots 142 of the cap 140. When the cap is “locked”, meaningthe teeth 148 have engaged the teeth 108 of the outer sleeve 110, a userwill insert the tabs 346 into the openings of the angled slots 142. Theuser will then rotate the reducer extension 340 clockwise 90 degreesuntil the tabs 346 are locked within the angled slots 142. The threads344 of the reducer extension 340 engage the threads, not shown, of thereduction handle 330. The reduction handle 330 is a generallycylindrical in shape with a grip and an inner lumen that containsthreads. The distal end of the inner lumen is also dimensioned toreceive the proximal end of the reduction sleeve 330.

While the housing is still fixed in the guide assembly 100 and after theanchors have all been implanted in the target bone the user attaches thereducer extension 340 to the cap 140. Then the user slides the reductionsleeve 302 over the guide assembly, aligning the openings 320 to engagethe connecting element 14. Finally the user slides the reduction handle330 over the reducer extension and rotates the handle clockwise toengage the threads 344 and the threads within the reduction handle 330.The reduction handle 330 engages the proximal end of the reductionsleeve 302 and as the reduction handle 330 is rotated clockwise a forceis applied on the reduction sleeve 302 drawing the housing 18 andconnecting element 14 together.

While the invention is susceptible to various modifications andalternative forms, specific embodiments thereof have been shown by wayof example in the drawings and are herein described in detail. By way ofexample, an alternative guide assembly having opposing slots ofdifferent widths and/or without keyholes (as shown in FIG. 14) may beutilized to accommodate enlarged ends on some connecting elements. Inaddition, it is within the scope of the invention to introduce varioustypes of bone anchors, including but not limited to a pedicle or hookanchor. The description herein of specific embodiments is not intendedto limit the invention to the particular forms disclosed, but on thecontrary, the invention is to cover all modifications, equivalents, andalternatives falling within the spirit and scope of the invention.

What is claimed is:
 1. A guide assembly for introducing a bone anchor toan operative target site, comprising: an outer sleeve having a centralpassage extending from a distal end toward a proximal end of the outersleeve, the outer sleeve configured for releasably engaging the boneanchor; an inner sleeve housed at least partly in the central passageand movable relative to the outer sleeve; and a locking mechanism,wherein the locking mechanism, when engaged, prevents movement of theinner sleeve in a first direction thereby preventing releasing of thebone anchor from the guide assembly, wherein the inner sleeve comprises:a main sleeve and a sleeve extension that are translationally fixed androtatable relative to each other, and wherein an outer surface of thesleeve extension includes a thread along at least a portion thereof andengaged with a complementary thread formed in an inner surface of theouter sleeve such that rotation of the sleeve extension moves the innersleeve along a longitudinal axis relative to the outer sleeve, the outersleeve being engaged with the main sleeve to prevent rotation of themain sleeve relative to the outer sleeve.
 2. The guide assembly of claim1, wherein the locking mechanism, when engaged, further permits movementof the inner sleeve in a second direction opposite the first directionthereby permitting affixing the bone anchor to the guide assembly. 3.The guide assembly of claim 1, wherein the locking mechanism comprises afirst set of teeth fashioned on the proximal end of the outer sleeve anda second set of teeth complementary to the first set of teeth andfashioned on the inner sleeve facing the first set of teeth.
 4. Theguide assembly of claim 3, wherein the first set of teeth or the secondset of teeth includes a generally perpendicular face and a sloped face.5. The guides assembly of claim 1, wherein the locking mechanismautomatically engages when the inner sleeve moves toward or near thedistal end of the outer sleeve.
 6. The guide assembly of claim 1,wherein the locking mechanism is disengaged when the inner sleeve movesproximally relative to the outer sleeve.
 7. The guide assembly of claim1, wherein at least a portion of the inner sleeve extends beyond theproximal end of the outer sleeve.
 8. The guide assembly of claim 1,wherein the inner sleeve further comprises a cap longitudinally moveablerelative to the sleeve extension and a biasing element that biases thecap distally.
 9. A guide assembly for introducing a bone anchor to anoperative target site, comprising: an outer sleeve having a centralpassage extending from a distal end toward a proximal end of the outersleeve, the outer sleeve configured for releasably engaging the boneanchor; an inner sleeve housed at least partly in the central passageand movable relative to the outer sleeve; and a locking mechanism,wherein the locking mechanism, when engaged, prevents movement of theinner sleeve in a first direction thereby preventing releasing of thebone anchor from the guide assembly, wherein the inner sleeve comprises:a main sleeve and a sleeve extension that are translationally fixed androtatable relative to each other, wherein the inner sleeve furthercomprises a cap longitudinally moveable relative to the sleeve extensionand a biasing element that biases the cap distally, and wherein the capand at least a part of the sleeve extension extends beyond the proximalend of the outer sleeve.
 10. The guide assembly of claim 1, wherein theinner sleeve is housed only partly in the central passage and movablerelative to the outer sleeve along a longitudinal axis; and wherein thelocking mechanism, when engaged, prevents proximal movement of the innersleeve thereby preventing releasing of the bone anchor from the guideassembly and permits distal movement of the inner sleeve therebypermitting affixing the bone anchor to the guide assembly, wherein thelocking mechanism is configured to be disengaged by movement of theinner sleeve proximally relative to the outer sleeve.
 11. The guideassembly of claim 10, wherein the locking mechanism comprises a firstset of teeth fashioned on the proximal end of the outer sleeve and asecond set of teeth complementary to the first set of teeth andfashioned on the inner sleeve facing the first set of teeth.
 12. Theguide assembly of claim 11, wherein the first set of teeth or the secondset of teeth includes a generally perpendicular face and a sloped face.13. The guide assembly of claim 10, wherein the locking mechanismautomatically engages when the inner sleeve moves toward or near thedistal end of the outer sleeve.
 14. The guide assembly of claim 10,wherein the at least a portion of the inner sleeve extends beyond theproximal end of the outer sleeve.
 15. The guide assembly of claim 10,wherein the inner sleeve further comprises a cap longitudinally movablerelative to the sleeve extension and a biasing element that biases thecap distally.
 16. A guide assembly for introducing a bone anchor to anoperative target site, comprising: an outer sleeve having a centralpassage extending from a distal end toward a proximal end of the outersleeve, the outer sleeve configured for releasably engaging the boneanchor; an inner sleeve housed at least partly in the central passageand moveable relative to the outer sleeve, the inner sleeve having adistal end and a proximal end; and a locking mechanism, wherein thelocking mechanism, when engaged, prevents movement of the inner sleevefrom the locked position thereby preventing releasing of the bone anchorfrom the guide assembly, wherein the inner sleeve is moveable along alongitudinal axis between an open position and a locked position fortemporarily fixing a housing to the guide assembly, the distal end ofthe inner sleeve including a first distal extension having a firstlongitudinally extending engagement feature on the bone anchor, andwherein the inner sleeve comprises a biasing element for biasing atleast a portion of the inner sleeve distally.